There mere fact that you work 'off the books' doesn't mean that you don't have the legal obligation to pay taxes on your earnings. If IRS catches you, the penalties for failing to report are yours.
IRS agents have a variety of ways to catch people in the "cash" situation. They can actually do a "net worth" audit that starts with a premise of what you had at a certain point in time, and then show what you have now.
Income received for babysitting, housecleaning and lawn cutting are all examples of taxable income, even if less than $600.
Use Form 1040, Schedule C, Profit or Loss from Business, to report income and expenses. Expenses must be a reasonable amount to be deducted and shouldn’t be inflated. You will also need to prepare Form 1040 Schedule SE for self-employment taxes if the net profit exceeds $400 for a year. Do not report this income on Form 1040 Line 6 as Other Income.